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    Subject[PATCH 22/22] x86_64: use relative pointers with dynamic debug
    Date
    Similar to how x86_64 uses bug_entry-relative pointers to reduce
    sizeof(struct bug_entry), the same thing can now be done for struct
    _ddebug, saving 16 bytes for each of those (i.e., each pr_debug, dev_dbg
    etc. in a CONFIG_DYNAMIC_DEBUG kernel).

    Note the use of .ifndef/.endif in asm to avoid

    fs/aio.c:1382: Error: symbol `__UNIQUE_ID_ddebug112' is already defined

    This is due to uses of pr_debug et al in functions that get inlined. In
    such a case, __COUNTER__ is obviously only expanded once, but the asm()
    is repeated once for every inlined instance. Letting all instances share
    the same descriptor object is the right thing to do; that is also what
    happens when it is the compiler that defines a static object inside an
    inline(d) function.

    Cc: x86@kernel.org
    Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
    ---
    arch/x86/Kconfig | 1 +
    arch/x86/include/asm/dynamic_debug.h | 35 +++++++++++++++++++++++++++++++++++
    2 files changed, 36 insertions(+)
    create mode 100644 arch/x86/include/asm/dynamic_debug.h

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    index 1a0be022f91d..a44168930e52 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -90,6 +90,7 @@ config X86
    select CLOCKSOURCE_WATCHDOG
    select DCACHE_WORD_ACCESS
    select DMA_DIRECT_OPS
    + select DYNAMIC_DEBUG_RELATIVE_POINTERS if X86_64
    select EDAC_ATOMIC_SCRUB
    select EDAC_SUPPORT
    select GENERIC_CLOCKEVENTS
    diff --git a/arch/x86/include/asm/dynamic_debug.h b/arch/x86/include/asm/dynamic_debug.h
    new file mode 100644
    index 000000000000..350ef6e2feff
    --- /dev/null
    +++ b/arch/x86/include/asm/dynamic_debug.h
    @@ -0,0 +1,35 @@
    +/* SPDX-License-Identifier: GPL-2.0 */
    +#ifndef _ASM_X86_DYNAMIC_DEBUG_H
    +#define _ASM_X86_DYNAMIC_DEBUG_H
    +
    +#ifdef HAVE_JUMP_LABEL
    +# ifdef DEBUG
    +# define _DPRINTK_ASM_KEY_INIT ASM_STATIC_KEY_INIT_TRUE
    +# else
    +# define _DPRINTK_ASM_KEY_INIT ASM_STATIC_KEY_INIT_FALSE
    +# endif
    +#else
    +# define _DPRINTK_ASM_KEY_INIT ""
    +#endif
    +
    +#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt) \
    + extern struct _ddebug name; \
    + asm volatile(".ifndef " __stringify(name) "\n" \
    + ".pushsection __verbose,\"aw\"\n" \
    + "1:\n" \
    + __stringify(name) ":\n" \
    + "\t.long %c0 - 1b \t# _ddebug::modname_disp\n" \
    + "\t.long %c1 - 1b \t# _ddebug::function_disp\n" \
    + "\t.long %c2 - 1b \t# _ddebug::filename_disp\n" \
    + "\t.long %c3 - 1b \t# _ddebug::format_disp\n" \
    + "\t.long %c4 \t# _ddebug::flags_lineno\n" \
    + "\t.long 0 \t# <padding>\n" \
    + _DPRINTK_ASM_KEY_INIT \
    + ".popsection\n" \
    + ".endif\n" \
    + : : "i" (KBUILD_MODNAME), "i" (__func__), \
    + "i" (__FILE__), "i" (fmt), \
    + "i" (_DPRINTK_FLAGS_LINENO_INIT))
    +
    +#endif /* _ASM_X86_DYNAMIC_DEBUG_H */
    +
    --
    2.16.4
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